This invention relates to the field of electrocoating metal substrates. More particularly, the invention relates to improved electrocoating materials and improved methods for use in electrocoating systems for electrodepositing a coating on interior electrically-conductive surface areas of metal substrates such as container bodies and components.
Basically, electrocoating is the electrodeposition of organic resinous coating materials on electrically-conductive surface areas, from polyelectrolytic electrocoating material mediums which for purposes of this invention are anodic or cathodic, aqueous base bath solutions, suspensions or dispersions. The electrocoating mediums ultimately contain coating ions or polyelectrolytic particles, which, in the case of anodic mediums, carry a negative charge in the bath and when a voltage is applied and current is induced to flow through the medium, migrate to and discharge onto any positively charged surface of a metal substrate, i.e., the anode, which may be in contact with the medium. Conversely, the polyelectrolytic particles, in the case of cathodic mediums, carry a positive charge in the medium, and, upon application of a voltage, migrate to and discharge onto any negatively charged surface of a metal substrate, i.e. the cathode, which may be in contact with the medium.
A layer of particulate coating material is electrodeposited adjacent the electrically charged metal substrate as the direct current flows between it and an oppositely electrically charged electrode such as a wire or rod, immersed in the coating bath. The process is driven by an electrical potential which can typically range from about 50 to 500 volts. The electrodeposition of the coating material takes place only at electrically-conductive surface areas of the metal object because only at such areas is there an electrical circuit and the electrical action which allows the flow of direct current needed to cause the polyelectrolytic particles to be electrodeposited adjacent the electrically-conductive surface.
The thickness of the layer of particulate material electrodeposited is automatically regulated by the electrical conductivity of the particular mediums used. Once a certain layer thickness of coating material has attached to the electrically-conductive surface area of the metal substrate, the electrodeposited coating material, in having a low electrical conductivity, increasingly tends to insulate the surface area from the coating bath in which it is immersed, transforming it into a non-conductive surface, whereby direct current flow therein greatly diminishes and eventually ceases, with the resulting inhibition of further electrodeposition of coating material.
One particular field where it has been found desirable to coat metal substrates is in the manufacturing of metal containers, cans or components thereof, where it is necessary that all the exposed, uncoated surface areas of the metal can be coated to protect the metal from corrosion.
A typical method which has been used for coating and perfecting coverage thereof on the interior surfaces of metal can bodies such as used in the packaging of beer or carbonated beverages, is to employ a double coat system which involves initially applying a base coat as by roller onto metal stock while in the flat and, after fabricating the can from the coated stock, applying a second or top coat rolled or sprayed onto the interior of the fabricated can or component of seal any discontinuities or other electrically porous or conductive areas in the base coat. The top coat is an overall coat since location of discontinuities is usually not reliably ascertainable.
Recent electrocoating technology has made it possible to eliminate the top coat and to apply in its stead a repair coating only adjacent the discontinuities, or to replace the base and top two-coat system with only a single, full coating.
Since metal containers are usually relatively low cost items, commercially, they must be manufactured at high speeds. Protective organic coatings applied to the interiors of the containers, especially those for packaging food, beer, beverages and like products must be applied at high speeds and must be of very high quality. The containers and coatings thereon are frequently cycled at high temperatures and they often must protect the metal surfaces from corrosive container contents while permitting little or no change or affect in the contents themselves.
Conventional aqueous electrocoating materials used for coating such containers are usually manufactured and shipped as an aqueous concentrate comprising an organic resin such as a carboxylic acid resin, having at least some of its reactive sites neutralized, usually by an amido compound. These materials when shipped usually contain from about 20 to 60 percent resin solids which already or later are made water soluble. Before or during use in electrocoating processes, water, usually de-ionized or distilled, is added to the concentrates to dilute and disperse them into the water dispersion medium.
Heretofore, these aqueous electrocoating materials and mediums have not, for commercial practice, been entirely satisfactory for obtaining the desired high quality coatings on the previously mentioned and like high standard containers and components. One major shortcoming of the coating materials is their insufficient "throwing power", i.e. their inability to evenly, uniformly and sufficiently coat cracks, curves, seams and other container and/or component areas remote from the throwing electrode, as adequately as areas adjacent thereto. One reason for inadequate throwing power is the materials' low initial and/or diminishing resistivity during use. Another major shortcoming is that their deposits or coatings upon metal surfaces form gas bubbles therein. During baking or curing of the coatings at high temperatures, the gases evolve leaving pin holes or voids which seriously affect the coating quality. Another shortcoming is that the materials do not sufficiently wet the metal substrate. This limits their ability to adhere to the substrate and to coat remote areas thereof.
Improving throwing power and wetting ability by slowing coating time is not commercially practical, and increasing voltage to increase throwing power gives non-uniform coatings adjacent to as compared to areas remote from the throwing electrode. The above shortcomings often render it difficult to provide container coatings which obtain quick test, iron pick-up and other test values sufficient to qualify them for use on the aforementioned high-standard containers.
It has now been found that according to this invention the aforementioned shortcomings, problems and disadvantages are overcome. Utilizing conventional operating conditions, improved coatings are obtained employing the aqueous electrocoating methods and materials or mediums of this invention. The coatings obtained have the same or increased resistivity, increased throwing power, and more even, more uniform and thicker overall coatings. Wetting and adhesion to the metal substrate is improved, and importantly, the coatings and deposits obtained are more pliable. This releases more entrapped gases and leaves less pin holes and voids after curing. Further, higher quality coatings with lower quick test, iron pick-up and other test values are provided.
These and other advantages are achieved with the improved electrocoating methods and improved aqueous electrocoating materials of this invention which generally involve adding with agitation, to obtain presence in the materials, of an alcohol having from 1 to 8 carbon atoms.